This fix:
6029211
capitalized all environment variables on Windows. It can be illustrated by this short program:

import subprocessprint(subprocess.check_output(    "set | findstr /c:SystemRoot /i", shell=True, universal_newlines=True))import gitprint(subprocess.check_output(    "set | findstr /c:SystemRoot /i", shell=True, universal_newlines=True))

The output is:

SystemRoot=C:\WindowsSYSTEMROOT=C:\Windows

This side effect breaks our use case currently. We use gnu make in cygwin for our build, in which all environment variables are case sensitive.

The core problem wasunittest.mock.patch.dict(os.environ, {"NoDefaultCurrentDirectoryInExePath": "1"}) -- in which it will try to treatos.environ as a dictionary, butos.environ is not just a simple dictionary. It actually remembers the original casing of the environment variable. Unfortunately when reading it as dictionary it capitalize all letters.

We can also observe the same side effect with this code below:

import osimport subprocessimport unittest.mockprint(subprocess.check_output(    "set | findstr /c:SystemRoot /i", shell=True, universal_newlines=True))with unittest.mock.patch.dict(os.environ, {"NoDefaultCurrentDirectoryInExePath": "1"}):    passprint(subprocess.check_output(    "set | findstr /c:SystemRoot /i", shell=True, universal_newlines=True))

The side effect is the same as above.